staging: comedi: ni_mio_common: clarify the cmd->start_arg validation and use

Clarify the cmd->start_arg validation in Step 3 of the (*do_cmdtest)
functions.

For a TRIG_INT source, the cmd->start_arg is actually the valid
trig_num that is used by the async (*inttrig) callbacks.

Refactor the (*inttrig) functions so that the cmd->start_arg is used
to check the trig_num instead of the open coded values.

For aesthetics, refactor the (*do_cmd) functions to use if/else instead
of the switch to handle the cmd->start_src.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>